PLIOCENE STRATIGRAPHY AND GEOLOGY OF THE NORTHEASTERN ILERET REGION, KENYA by Casey Lee Kidney A thesis submitted to the faculty of The University of Utah in partial fulfillment of the requirements for the degree of Master of Science in Geology Department of Geology and Geophysics The University of Utah December 2012 Copyright © Casey Lee Kidney 2012 All Rights Reserved The University of Utah Graduate School STATEMENT OF THESIS APPROVAL The thesis of ________________________Casey Lee Kidney_______________________ has been approved by the following supervisory committee members: _____________ Francis H. Brown______________ , Chair October 22, 2012 Date Approved Ronald L. Bruhn , Member October 22, 2012 Date Approved Thure E. Cerling , Member October 22, 2012 Date Approved and by _____________________D. Kip Solomon_____________________ , Chair of the Department of __________________ Geology and Geophysics_________________ and by Charles A. Wight, Dean of The Graduate School. ABSTRACT Five members of the Koobi Fora Formation: the Lonyumun, Moiti, Lokochot, Tulu Bor, and upper Burgi members, are exposed in Areas 40 and 41 (study area) northeast of Ileret in northern Kenya. Areas 40 and 41 were first mapped using tonal contrasts on aerial photographs by Key and Watkins in their 1988 study, and were not revisited until Gathogo and Brown did a reconnaissance, broadly mapping exposures in their 2006 study. The study area is located on the eastern margin of the Turkana Basin, where the base of the Koobi Fora Formation is in contact with volcanic rocks of Miocene age. Five tuffs are exposed in the study area. Four tuffs occur in the Lonyumun Member: the Guo Tuff, the Kanyeris Tuff, and the newly named Tukunan and Kisemei tuffs. This study, directly or indirectly provides dates for the tuffs in the Lonyumun Member and associated strata by means of magnetostratigraphy and 40Ar/39Ar dating methods. Paleomagnetic sampling and the known dates on the Moiti and Tulu Bor Tuffs of 3.4 and 4.0 Ma respectively, proved invaluable in assigning ages to strata, and was materially helped by an 40Ar/39Ar age of 4.06 ± 0.02 Ma on the Kisemei Tuff. Three tuffs known at many other localities south of the study area, the Lokochot, a-Tulu Bor and Burgi Tuffs, are missing from the study area. Stratigraphic correlations between exposures proved difficult due to numerous and pervasive disconformable surfaces, so that significant time gaps exist in the Lonyumun Member; all of the Moiti Member except the Moiti Tuff is missing, as is all of the lower Burgi Member. Newly discovered fossils in the Lonyumun Member have also been indirectly dated. Initial identification suggests correlation to fossils in the Apak Member at Lothagam southwest of Lake Turkana. In Areas 129 and 130 the Tulu Bor Member is in contact with the Lonyumun Member. Three tuffs, disconformable on the Tulu Bor Tuff allow correlation outside of these areas of which the Aberegaiye Tuff is dated to 2.70 ± 0.02 Ma on alkali feldspars contained in pumices. The other tuffs include a correlate to a tuff in submember D-5 of the Shungura Formation and a correlate to Area 116 to a tuff below the Ninikaa Tuff, the former being the first documented proof for deposition between 2.6-2.0 Ma in the Koobi Fora Formation. iv TABLE OF CONTENTS ABSTRACT...........................................................................................................................Ill LIST OF FIGURES.............................................................................................................. vll LIST OF TABLES..................................................................................................................x ACKNOWLEDGEMENTS..................................................................................................xl Chapters 1 INTRODUCTION............................................................................................................ 1 1.1 Geography and Climate of the Study Area..................................................... 2 1.2 Previous Work in the Study A rea.................................................................... 3 2 REGIONAL GEOLOGIC SETTING.............................................................................8 2.1 Structural - Tectonlc Settlng............................................................................8 2.2 Stratlgraphlc Settlng........................................................................................ 10 3 OBJECTIVES................................................................................................................ 13 4 METHODS......................................................................................................................14 4.1 Fleld W ork.......................................................................................................14 4.2 Laboratory Analysls........................................................................................ 15 5 STRATIGRAPHIC DESCRIPTIONS AND INTERPRETATIONS........................19 5.1 Lonyumun Member........................................................................................ 21 5.2 Moltl Member..................................................................................................33 5.3 Undlfferentlated Lokochot and Tulu Bor Members.....................................36 5.4 Tulu Bor Member........................................................................................... 37 5.5 Burgl Member ..................................................................................................40 6 AREAS 129, 130, 135, AND 139.................................................................................75 6.1 Lonyumun Member........................................................................................ 76 6.2 Tulu Bor Member........................................................................................... 76 6.3 Burgi Member..................................................................................................78 6.4 KBS Member...................................................................................................79 6.5 Okote Member.................................................................................................80 7 MAGNETOSTRATIGRAPHY.................................................................................... 89 7.1 Reverse Polarity interval in the early Gilbert Chron (4.49-4.29)................90 7.2 Normal Polarity of the Cochiti Subchron (4.29-4.18)..................................92 7.3 Reverse Polarity in the late Gilbert Chron (4.18-3.59)............................... 92 7.4 Normal Polarity interval in the early Gauss Chron (3.59-3.33)..................93 8 GEOLOGY OF FOSSIL OCCURENCES................................................................... 97 8.1 Vertebrate Fossils........................................................................................... 97 8.2 Invertebrates, Plants, and Diatoms................................................................. 98 9 GEOLOGIC STRUCTURES...................................................................................... 102 10 DISCUSSION.............................................................................................................. 106 11 CONCLUSIONS.......................................................................................................... 109 11.1 Stratigraphy...................................................................................................110 11.2 Magnetostratigraphy....................................................................................110 11.3 Ages of Volcanic Tephra.............................................................................111 11.4 Disconformities............................................................................................ 112 11.5 Fossil Assemblage in Area 40..................................................................... 113 11.6 Structures......................................................................................................113 11.7 Future Work..................................................................................................115 APPENDICES A. GEOLOGIC MAP............................................................................................. 118 B. SAMPLE LOCATIONS....................................................................................120 C. FELDSPAR 40Ar/39Ar DATING RESULTS................................................... 125 D. PALEOMAGNETIC MEASUREMENTS.......................................................127 E. PALEOMAGNETIC SAMPLE LOCATIONS AND STRATIGRAPHIC POSITIONS............................................................................................................. 155 REFERENCES.....................................................................................................................159 vi LIST OF FIGURES Figures Page 1. Overview of the Ileret area showing major geographical features.............................. 7 2. Overview of the East African Rift System (EARS)................................................... 12 3. General stratigraphic column for strata and marker horizons in study area and related areas to the south........................................................................................................... 48 4. Correlated stratigraphic sections in the study area..................................................... 49 5. Locations of stratigraphic sections in the study